The technology for the production of aluminium by the electrolysis of alumina, dissolved in molten cryolite containing salts, at temperatures around 950° C. is more than one hundred years old. This process and the cell design have not undergone any great change or improvement and carbonaceous materials are still used as electrodes and cell linings.
Using metal anodes in aluminium electrowinning cells would drastically improve the aluminium process by reducing pollution and the cost of aluminium production. Many patents have been filed on non-carbon anodes but none has found commercial acceptance, also because of economical reasons.
Several designs for oxygen-evolving anodes for aluminium electrowinning cells were proposed in the following documents. U.S. Pat. No. 4,681,671 (Duruz) discloses vertical anode plates or blades operated in low temperature aluminium electrowinning cells. U.S. Pat. No. 5,310,476 (Sekhar/de Nora) discloses oxygen-evolving anodes consisting of roof-like assembled pairs of anode plates. U.S. Pat. No. 5,362,366 (de Nora/Sekhar) describes non-consumable anode shapes, such as roof-like assembled pairs of anode plates. U.S. Pat. No. 5,368,702 (de Nora) discloses vertical tubular or conical oxygen-evolving anodes for multimonopolar aluminium cells. U.S. Pat. No. 5,683,559 (de Nora) describes an aluminium electrowinning cell with oxygen-evolving bent anode plates which are aligned in a roof-like configuration facing correspondingly shaped cathodes. U.S. Pat. No. 5,725,744 (de Nora/Duruz) discloses vertical oxygen-evolving anode plates, preferably porous or reticulated, in a multimonopolar cell arrangement for aluminium electrowinning cells operating at reduced temperature.
U.S. Pat. No. 5,938,914 (Dawless/LaCamera/Troup/Ray/Hosler) describes an aluminium electrowinning cell having vertical inert anodes interleaved with vertical cathodes. The anodes are covered with an angled roof which diverts anodically evolved oxygen bubbles to agitate the cell's molten electrolyte.
WO01/31088 (de Nora) discloses aluminium electrowinning cells with solid anodes having a V-shaped active surface facing sloping cathodes. The anodes and cathodes are associated with vertical passages for the circulation of alumina-rich electrolyte to a bottom part of the inter-electrode gaps spacing the anodes and cathodes.
WO00/40781 and WO00/40782 (both de Nora) both disclose aluminium production anodes with a series of coplanar parallel spaced-apart elongated anode members which are electrochemically active for the oxidation of oxygen. The anodes disclosed in WO00/40781 are fitted with a series of inclined baffles promoting the circulation of electrolyte through the anodes and are designed for use with a cathode surface that is horizontal or at a small angle as disclosed in WO01/31086 (de Nora/Duruz).
In WO00/40782 the electrochemically active anode surface may be substantially vertical, the horizontal anode members being spaced apart one above the other, for example like venetian blinds next to a substantially vertical cathode. In particular, two downwardly converging spaced apart adjacent anodes can be arranged between a pair of substantially vertical cathodes. The adjacent anodes are spaced apart by an electrolyte down-flow gap in which alumina-rich electrolyte flows downwards until it circulates via the adjacent anodes' flow-through openings into the inter-electrode gaps.